The present invention relates to a capacitance liquid level or volume sensing circuit, in combination with a capacitance touch control circuit which may be used to actuate peripheral equipment only in response to the sensing of a minimum liquid level or volume.
Various liquid level or volume sensing systems are known. Some are primarily mechanical in nature, and generally utilize a float which moves up and down on the surface of liquid in a vessel as the level and volume of the liquid change. The float may be mechanically connected with a gauge for providing a visual indication of the level of liquid, or with a valve for the liquid to maintain a selected level of liquid in the vessel.
In another type of liquid level sensor adapted for use where a reading of the liquid level is to be generated at a location remote from the liquid, a float which follows the level of liquid is moved along a potentiometer. The potentiometer generates a signal in accordance with the position of the float, and the signal is carried over conductors to the remote location, whereat it is used to provide an indication of the liquid level
Because of the mechanical elements involved, the above types of sensors are prone to breakage and wear, resulting in inaccurate liquid level indications and necessitating replacement of the sensors. Also, since the readings depend upon movement of mechanical elements, it is difficult to calibrate the sensors and inaccuracies often result.
One use for liquid level sensors not heretofore generally recognized is with water jetted bathtubs and spas, which are gaining increasing popularity. In order that a pump for recirculating the water not be destroyed if insufficient water is in the bathtub or spa, operation of the pump should be inhibited unless there is at least a minimum level of water. However, that requires use of a liquid level sensor, and conventional mechanical sensors are ill suited for the purpose, since not only would they be subject to breakage upon contact by users, but they would also present a physical hazard upon contact.
In addition to not operating the pump in the absence of a sufficient level of water, to conserve energy, water should be recirculated only when the bathtub or spa is being used and, for convenience, the user should be able to control operation of the pump while he is in the spa or bathtub. Although a simple switch for the pump could be used, because of the high voltages normally present and the fact that the user would be immersed in water and likely operate the switch with a wet hand, the technique would be inherently hazardous and possible fatal.
As an alternative to a conventional switch, a capacitance switch including a touch plate or sensor, operated at a very low voltage and current, could be used. However, such capacitance switches as are presently available are unstable, unreliable and require on-site manual sensitivity adjustment to properly respond only to the change in capacitance that occurs when the sensor is touched. Systems requiring critical manual adjustment generally cannot compensate for changing conditions and drift caused by naturally occurring environmental conditions, particularly those which would occur in the inherently wet environment of a bathtub or spa, and therefore can be reliably used only in controlled environments.